This invention relates to rotary positive displacement fluid meters, and more particularly, it relates to rotary meters which measure the flow of gas.
Rotary meters are used for commercial, residential and industrial buildings, as well as other locations to measure the amount gas which is consumed. Public utilities and other providers of gases use the measurements to bill customers. Public utility commissions in many states require the overall accuracy of all gas meters to be approximately one hundred percent with a permissible error of .+-.1% at all gas flow rates. Since the operation of various components within gas meters and gas meter installations make inaccuracies inherent, the designers and manufacturers of gas meters are faced with minimizing these effects.
Rotary positive displacement meters measure the flow of fluids such as natural gas through the use of one or more impellers installed in an impeller cavity located in a meter casing. The impeller cavity is in the path of the flow of gas through the casing. Thus, the flow of gas through the path rotates the impeller(s). The single or multiple impellers used in rotary meters normally have what is called a "figure-eight" or an "hour-glass" shape. As gas flows through the casing, the impeller(s) trap a known quantity of gas in a measuring chamber which is formed between the impeller(s) and an inner wall of the impeller cavity. Each total revolution of an impeller causes a number of these known quantities of gas to be accumulated in the measuring chamber and passed through the rotary meter. The number of rotations of the impellers is counted on a register and totalled to determine the amount of gas flowing through the rotary meter.
A cross-section of a typical prior art rotary positive displacement meter 20 is shown in FIG. 1A-1D. The rotary meter 20 includes a casing or housing 22 having a fluid flow conduit 24 through which fluid is passed into the meter, which is measured and is passed out of the meter. The fluid flow conduit 24 includes a flow inlet channel 26 for gas intake, an inlet expansion chamber 28, an impeller cavity 30, an outlet expansion chamber 32 and an outlet channel 34 for gas expulsion. The impeller cavity 30 has an inner wall 33 extending around its periphery.
A pair of identical, figure-eight shaped impellers 36 and 38 is mounted within the impeller cavity 30. For the convenience of following the rotation of the impellers in FIG. 1A-1D, the ends of the impeller 36 are marked A and B, while the ends of impeller 38 are marked C and D. The impellers 36 and 38 are constructed to rotate within the impeller cavity so that their ends pass in close proximity to the inner wall 33.
The pressure drop across the gas meter 20 as gas is consumed downstream of the meter 20 causes the upper impeller 36 to rotate in a clockwise direction, while the lower impeller 38 is rotated in a counter-clockwise direction. As explained above, in the course of this rotation, the figure-eight shaped impellers 36 and 38 each periodically forms a closed measuring chamber of a known volume between itself and a portion of the inner wall 33. As a result, the rotating impellers 36 and 38 separate the flowing gas stream into a series of pulses of a known volume of gas as the gas passes through the meter 20. This operation of the rotary meter 20 is shown in FIG. 1A through 1D and explained as follows:
Referring first to FIG. 1A, as the bottom impeller 38 rotates in a counter-clockwise direction toward the horizontal position, gas enters from the inlet channel 26 and the inlet expansion chamber 28 to a space between the lower impeller 38 and the inner wall 33 of the impeller cavity 30. Upper impeller 36 prevents gas from flowing from the input expansion chamber 28 and through the impeller cavity 30. A previously trapped pulse of gas is being released from between impeller 36 and a portion of innerwall 33 toward exit expansion chamber 32.
When impeller 38 reaches the horizontal position shown in FIG. 1B, a known volume of gas is trapped in a measuring chamber 40 between impeller 38 and the inner wall 33. Impeller 36 is in the vertical position and continues to block the flow of gas from beyond the impeller cavity 30.
As the impeller 38 continues to turn in the counter-clockwise direction, FIG. 1C shows the volume of fluid trapped in the measuring chamber 40 is discharged as a pulse through the outlet expansion chamber 32 and the outlet channel 34. Gas from the inlet expansion chamber 28 is now becoming trapped in a closing space between the top impeller 36 and the inner wall 33. Impeller 38 prevents additional gas from flowing from the inlet channel 26 and inlet expansion chamber 28 through the meter 20.
FIG. 1D shows that as the impeller 36 rotates in the clockwise direction it reaches its horizontal position at which it confines another known volume of fluid in a measurement chamber 42 between it and a portion of the inner wall 33. The measuring chamber 42 is the same size as the measuring chamber 40. The balance of the gas that was in the measurement chamber 40 is discharged from the meter 20, as the impeller 38 continues to prevent additional gas from flowing from the inlet expansion chamber 28. The gas in the measurement chamber 42 is then discharged as a pulse as the impeller 36 continues to rotate toward the position shown in FIG. 1A.
This process is repeated twice for each complete revolution of the impellers 36 and 38. During each rotation, the upper impeller 36 fills and empties its measurement chamber twice, as does the lower impeller 38. Ideally, no gas will pass from the inlet connection to the outlet connection without being trapped in the measurement chambers 40 and 42.
While rotary meters are of great utility, the pressure pulses discussed above are felt throughout the length of the meter case as well as the whole meter installation. These pressure pulses lead to meter inaccuracies. Each complete rotation of the impellers 36 and 38 produces four pressure pulses in the gas stream as the measurement chambers 40 and 42 each fill and empty twice during each rotation of the impellers. Therefore, the frequency of the pulsations will be four times the frequency of the rotations of the impellers 36 and 38. As the gas flow rate increases, rotation rates of the impellers 36 and 38 increase, gas pressure pulses are more frequent, and inaccuracies due to gas pressure pulses become more significant.
Prior to this invention, rotary gas meters having inherent inaccuracies due to gas pressure pulses were installed to measure the flow of gas. One U.S. Pat. No. 5,207,088 dealt with the effects of these pulses by eliminating accuracy errors caused by standing waves of sound produced by these pulses in configurations used for testing the accuracy of the rotary gas meter. Where a rotary meter to be tested was connected to a prover master meter by a conduit, the testing configuration included at least one expansion chamber having a cross-sectional area at least seven times as great as the cross-sectional area of the conduit. This expansion chamber reportedly eliminated the standing waves produced by gas pressure pulses at various harmonics of the frequency of these pulses as the flow rate of gas increased during the testing of the rotary meter. However, this expansion chamber was not used once the testing of meter accuracy was completed during initial calibration. The effects of gas pulses on the accuracy of a rotary meter after it was installed were left unchecked and unchanged.
Additionally, it is often desirable to attempt to reduce the cost of a rotary meter by manufacturing its casing through the use of an extrusion process. During the extrusion process, an ingot of metal from which the meter casing is to be manufactured is driven through a die to form the casing. Referring to the meter casing 22 shown in FIG. 1A, the hollow area comprising the impeller cavity 30, the inlet expansion chamber 28, and the outlet expansion chamber 32 is formed by inserting a core plunger having the shape of the cross-section of these hollow areas into a heated ingot before the ingot is driven through the die which forms the casing. Because substantial pressures are produced during the extrusion process, the cross-sectional area of the core plunger must be of a size to resist being damaged as the ingot is being driven through the dye.
Furthermore, the cost of a meter casing can be further reduced when the hollow areas in the casing are larger. Larger hollow areas result in the use of less material, thus reducing the cost of the meter casing. However, to be discussed later, there are practical limitations on how large one may make the flow conditioning chamber. For this reason it is desirable to have the hollow areas of the casing as large as practicable for the design of a rotary meter.